Switch mode power supplies or switching regulators, also referred to as DC to DC converters, are often used to convert an input supply voltage to an output voltage at a voltage level appropriate for the internal circuitry of an integrated circuit. For example, a 5 volts supply voltage provided to an integrated circuit may need to be reduced to 2.8 volts on the IC chip to operate the internal circuitry on the chip. A switching regulator provides power supply function through low loss components such as capacitors, inductors, and transformers, and power switches that are turned on and off to transfer energy from the input to the output in discrete packets. A feedback control circuit is used to regulate the energy transfer to maintain a constant output voltage within the desired load limits of the circuit.
A switching regulator can be configured to step up the input voltage or step down the input voltage or both. Specifically, a buck switching regulator, also called a “buck converter,” steps down the input voltage while a boost switching regulator, also called a “boost converter,” steps up the input voltage. A buck-boost switching regulator, or buck-boost converter, provides both step-up and step-down functions.
The operation of the conventional buck switching regulator is well known and is generalized as follows. A conventional buck switching regulator includes a pair of power switches which are turned on and off to regulate an output voltage to be equal to a reference voltage. More specifically, the power switches are alternately turned on and off to generate a switching output voltage at a switching output node, also referred to as the switch node. The switch node is coupled to an LC filter circuit including an output inductor and an output capacitor to generate an output voltage having substantially constant magnitude. The output voltage can then be used to drive a load.
More specifically, the pair of power switches is often referred to as including a “high-side power switch” and a “low-side power switch.” The high-side power switch is turned on to apply energy to the output inductor of the output filter circuit to allow the current through the inductor to build up. When the high-side power switch is turned off, the voltage across the inductor reverses and the current through the inductor reduces during this period. As a result, the inductor current ripples above and below the nominal output current. A relatively constant output voltage is maintained by the output capacitor. The low-side power switch is turned on and off for synchronous control operation.
FIG. 1 is a schematic diagram of a conventional buck switching regulator. Referring to FIG. 1, a switching regulator 1 includes a pair of power switches S1 and S2 configured to receive an input voltage VIN and are alternately turned on and off to generate a switching output voltage VSW at a switch node (SW) 22. The switching output voltage VSW is directly coupled to an LC filter circuit including an output inductor L1 and an output capacitor COUT to generate a regulated output voltage VOUT at a node 26 having a substantially constant magnitude. The output voltage VOUT can then be used to drive a load 30 whereby switching regulator 1 provides the load current ILOAD to maintain the output voltage VOUT at a constant level.
Switching regulator 1 includes a feedback control circuit to regulate the energy transfer to the LC filter circuit to maintain the constant output voltage within the desired load limits of the circuit. More specifically, the feedback control circuit causes power switches S1 and S2 to turn on and off to regulate the output voltage VOUT to be equal to a reference voltage VREF or to a voltage value related to the reference voltage VREF. In the present embodiment, a voltage divider including resistors R1 and R2 is used to divide down the output voltage VOUT which is then fed back to the switching regulator 1 as a feedback voltage VFB on a feedback node 28. The feedback voltage VFB is compared with the reference voltage VREF at an error processing circuit, such as an error comparator 12. The comparator output is coupled to a controller and gate drive circuit 14 to generate control voltages for the power switches based on a switching regulator control scheme. The control voltages are used to generate gate drive signals for the power switches S1 and S2.
Buck switching regulators or “buck regulators” with fixed on-time control are widely used in the industry for some important advantages as fast load transient response and easy control of a relatively large off-time and a very small fixed on-time to regulate a high input voltage to a low output voltage. Fixed on-time (or constant on-time) regulators are one type of voltage regulators employing ripple-mode control where the output voltage is regulated based on the ripple component in the output signal. Because of the switching action at the power switches, all switch-mode regulators generate an output ripple current through the switched output inductor. This current ripple manifests itself as an output voltage ripple due, principally, to the equivalent series resistance (ESR) in the output capacitor COUT placed in parallel with the load. The ESR of the output capacitor COUT is denoted as a resistor RESR in FIG. 1.
Recently, power systems including multiple power stages are used in applications that demand high output currents. Typically, these power systems operate under multi-phase control. For example, conventional multi-phase converters use a multi-phase PWM controller to generate clock signals with different phase shifts for each phase of the power stages. Control circuits for multi-phase converters are generally more complex and costly in implementation than their single-phase counterparts.